As is known in the transformation of plastic materials, a very important treatment is the process of dehumidification of the plastic material granules, in other words the removal of the water contained in the granules of so-called hygroscopic plastics.
The elimination of the humidity content of the granules is necessary because during the high temperature fusion of the material the water penetrates into the molecular chain of the polymer granulate and breaks it, which results in a reduction of mechanical performance and generates bubbles, pitting and uneven colouring of the product.
Traditional equipment for dehumidifying or separating and eliminating the water from the plastic granules are commonly known as dehumidifiers. In the majority of cases, a dehumidifier is a machine employing the principle of molecular filters. In practice, molecular filters constitute the principle component of the machine, which exploits their capacity for almost total adsorption at room temperature of the humidity of the air.
The dehumidified and thus dry air is then heated and passed through the granular material in a treatment container employing molecular filters. The granular material gradually yields up its humidity to the dry air passing through it. The duration of the process depends on the residual humidity of the material, and the temperature and flow of dry air.
The adsorption capacity of the filters is however quantitatively limited and after a certain time saturation is reached. At this point, the dehumidification process is interrupted and may be made to continue using a second container of filters, while the saturated filter container undergoes regeneration.
The majority of dehumidification plants therefore requires two containers of molecular filters operating in alternation.
Two kinds of filter containers are known, as shown in FIGS. 1 and 2.
FIG. 1 shows a filter container or tower C of cartridge type. In this configuration the air enters via a lower pipe I driven by a blower or compressor (not shown) and passes across an electrical resistance R which is inactive during process. The air then passes through a lower limiting grid GI beyond which it comes into contact with the mass of molecular filters MS.
Molecular filters capture and retain water molecules contained in the air being processed which thus becomes dried and after passing through the mass of filters MS crosses an upper limiting grid GS and enters an upper pipe S to be conveyed to a hopper (not shown) containing plastic granular material to be treated. The dried air flows through the granular material and is then recycled through the process.
In this process molecular filters MS of the first layer, i.e. the lowest filters first to be met by the incoming air, become quickly saturated so that humidity adsorption function is gradually taken over by the next following upper layer. When also this layer becomes saturated humidity adsorption takes place in the next following higher layer and so on, until the entire cartridge is saturated, as shown in FIG. 1a. 
The time required to reach saturation can be generally estimated as a function of the humidity adsorption capacity of the filters and the quantity of water to be eliminated, with an additional charge factor of about 30% to take into account local climatic conditions. Once the preset saturation time for the mass of filters MS has elapsed, the saturated filters are regenerated, i.e. they are subjected to a water removing process to be restored to their initial condition. To this end, controlled electrical resistance R is energized to heat the regeneration air from the lower pipe I to the temperature of about 300° C. required to break the electromagnetic bond binding the water molecules to the filter structure. Freed water molecules are removed by the regeneration air flowing from the bottom to the top of the device.
As noted above, the tower or container C is never completely saturated, and thus the water, before being removed through the top of the structure, is captured by the upper layers of filters which have not been saturated during processing phase until the same are heated to a sufficient extent by the flow of hot air flowing through them, thereby releasing any water molecules held in them. The phenomenon is repeated and gradually moved upwards until the entire mass of filters MS below the upper limiting grid GS in the tower C has been regenerated.
FIG. 2 diagrammatically shows a hollow cartridge tower CC. In order to exploit more uniformly the mass of filters MS in the tower, the an axial pipe TF extending through the filters is provided, which comprises a perforated grid to distribute air uniformly within the cartridge CC in both the lower and upper layers of the filter mass MS. The attainable results are slightly better than those achievable with the configuration of FIG. 1, the direction, course and changes in the air flows.
Clearly, in order to regenerate conventional filter cartridges or towers a considerable amount of energy and time are required. Most of the energy consumption in such towers is due to production of the heat required for regenerating the sieves. A serious operating problem of conventional dehumidifiers is the fact that such configurations are “orthogonal”, i.e. air is always conveyed along preferential routes in the sieves mass MS caused by the dynamics of the fluid in the tower. This means that it is impossible to exploit entirely the totality of the molecular sieve mass and hence one does not take full advantage of the potential of the tower, although one consumes a greater amount of energy than necessary, which considerably reduces the efficiency as a whole.